This invention relates generally to web transport systems and is particularly directed to a speed control for a web transport system wherein a web is advanced between a pair of rotating spools at speeds which include high speeds and very low speeds.
It has been proposed to control the speed of transport of a web from a supply spool to a take-up by providing a pair of servo generators, each driven by one of the spools, and continuously and algebraically summing the outputs of the servo generators with a derived control voltage which is proportional to the sum of the desired speeds of the two spools. The summed signal constitutes a motor drive signal which is selectively applied to a drive motor associated with the take-up spool, another motor associated with the supply spool remaining de-energized. Such a system is described in application Ser. No. 801,703, filed May 26, 1977, and assigned to the assignee of the present invention. Suffice it to say that such a system continuously reacts to variations in the speed of the spools so as to drive the spools at respective speeds whose sum is constant and proportional to the control voltage. As a result, the web is advanced between the spools at a maximum speed which is substantially less than the maximum speed of prior systems while yet advancing the entire web from the supply spool to the take-up spool in the same transport time as prior systems.
Although the above-described web transport system has been found to provide superior control over the speed of a web transport, it has been found that that system can be improved for operation at very low transport speeds at which the motion of the web is barely noticeable. This is particularly important in microfilm retrieval systems in which it is desirable to transport the film at high speeds to roughly locate a particular microfilm frame and to transport the film at very low speeds to more precisely position the frame in question.
At very low spool speeds, one fourth of a revolution per minute, for example, the gain of the servo control system becomes very low. To increase the over-all gain of the servo system to accommodate such low speeds would result in too much gain at high speeds and possible system instability.
Another problem which occurs at very low speeds is that the motors which drive the spools tend to rotate in a jerky motion. This is particularly noticeable when one of the motors operates at very low speeds in which its IR (current times resistance) drop is greater than its back EMF (electromotive force).
Accordingly, although the servo control system described in the above-identified application is satisfactory for low and high speeds of operation, the above-described problems render it less satisfactory, though operable, at very low speeds.